I would love for there to be a dungeon that is stuck in a pocket dimension or a void of some sorts.

The lore would revolve around some sort of anomaly that is affecting the perception of those with the echo. The echo leads you to a rift in our universe/dimension that is caused by an entity that is attempting to break through to our universe/dimension to extract revenge on Hydaelyn for imprisoning it there long ago (hence why the echo leads us there, to stop the same entity again).

When I imagined this dungeon I thought about a place that looks similar to this place: